In a wireless transit system in which vehicles mounted with batteries travel by receiving electric power from the batteries, it is necessary that the batteries be charged by ground charging equipment. Ground charging equipment is provided at a plurality of premises such as stations because the battery is limited in its capacity to store electricity. However, stoppage time of the vehicles at stations is short, so fast charging of batteries is demanded. Also, it is needed to detect SOC (State of Charge) of the batteries to control over adequate SOC thereof by preventing over charge or over discharge. However, as power supply from the batteries is needed even when the vehicle is making a stop, detection of battery SOC by measuring open-circuit voltage of the battery cannot be performed.
In document 1 (Japanese Laid-Open Patent Application No. 10-108380) is disclosed an apparatus and a method for charging a secondary battery for an electric car intended to make possible energy recovering with increased recovering efficiency under all environments. According to the invention of the document 1, charging power is controlled to be optimum by comparing DOD % (depth of discharge; ratio of discharged quantity of electricity to the rated capacity of battery) of the battery allowable for charging calculated from the temperature and voltage of the secondary battery with a DOD limit value allowable for charging memorized beforehand in a memory circuit, the DOD limit value being determined dependent upon temperature, DOD, and voltage of the secondary battery.
In document 2 (Japanese Patent Application No. 2006-003637) applied for by the applicant of the present invention is suggested a charging method in a wireless transit system, by which method voltage of the battery mounted on the vehicle is detected and charging is stopped when the detected voltage reaches a predetermined charge terminating voltage. However, as characteristic of internal resistance of battery varies depending on temperature of the battery, accurate estimation of batter SOC is difficult and errors occur under temperature variant environment.
According to an embodiment disclosed in FIG. 5 of document 2, a temperature control means is provided, and battery temperature is maintained to a temperature desirable for SOC of the battery. By this, control accuracy of SOC susceptible to temperature is improved, over charging is prevented, thereby curbing deterioration of battery.
According to document 1, charging power is controlled to be optimum by comparing DOD of the battery allowable for charging calculated from the temperature and voltage of the secondary battery with a DOD limit value allowable for charging determined dependent upon temperature, DOD, and voltage of the secondary battery memorized beforehand in a memory circuit, however, internal resistance of the battery changes depending on battery temperature variation and DOD (depth of discharge) allowable for charging varies according to the variation of internal resistance, so onerous control is needed taking temperature variation into consideration.
The method disclosed in document 2 has an advantage that influence of battery temperature variation is eliminated by providing the temperature control means for the battery, however, the provision of the battery temperature control device causes a problem of complication in structure.
There is known a method of estimating battery SOC by current integration. As charge and discharge of a battery is grasped by integral of battery current with respect to time (ampere hour) by integrating measured battery current by time. SOC of a battery mounted on a vehicle can be estimated by the current integration method, however, error of current measurement is accumulated and error of estimation of battery SOC increases as integration time increases.